extdom: make nss buffer configurable
    
    The get*_r_wrapper() calls expect a maximum buffer size to avoid memory
    shortage if too many threads try to allocate buffers e.g. for large
    groups. With this patch this size can be configured by setting
    ipaExtdomMaxNssBufSize in the plugin config object
    cn=ipa_extdom_extop,cn=plugins,cn=config.
    
    Related to https://fedorahosted.org/freeipa/ticket/4908
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>